For treating dental maladies a variety of different solutions are meanwhile on the market.
Generally, the dental maladies can be treated by restorative methods or prosthetic methods.
Prosthetic methods are typically used, if not sufficient remaining tooth structure is left which allows a restorative treatment, e.g. by filling a cavity with a dental filling material.
The prosthetic treatment usually starts with taking a dental impression from the dental situation in the mouth of the patient. The obtained record represents the status as is.
In a next step the tooth to be treated is further prepared, i.e. the tooth is shaped to a form which later allows the fixation of an artificial crown.
The artificial crown is typically designed from the information obtained from the dental impression and the shape of the treated tooth.
In a next step, the artificial crown is produced in a dental lab and specifically designed for this individual case. This procedure takes time and is expensive.
If, however, a fast and cheap prosthetic treatment is desired, the practitioner might consider using preformed crowns instead. This kind of treatment is often used in pediatric dentistry.
Different prefabricated crown types are meanwhile available on the market for this purpose.
The most commonly used solution is the stainless steel crown (e.g. from 3M Oral Care; 3M ESPE). Not only that stainless steel crowns are reasonable inexpensive to manufacture, they are also durable over years.
Preformed crowns made from stainless steel have in addition the benefit that they are pre-trimmed, belled and crimped for fast and easy placement. Due to a so-called “snap-on” feature, the stainless steel crown is readily retained and fits over the contour of the prepared tooth.
However, due to its metal surface, stainless steel crowns do not meet the desired aesthetic requirements.
To cure this defect, veneered stainless steel crowns have been suggested.
However, veneered stainless steel crowns show only a limited flexibility due to stiffer walls, at least at some areas of the crown. Moreover chipping of the veneering of these crowns is reported and a metallic shine of the underneath stainless steel crown lowers the esthetic appearance.
Thus, it was suggested to try to manufacture preformed crowns out of other materials.
Zirconia ceramic (ZrO2) is quite common for individually designed crowns for esthetic dentistry. ZrO2 has a couple of unique material properties, e.g. extremely high strength and toughness, translucency, stainability and biological compatibility, which makes it well suitable for crowns or even bridges. Therefore, it is no surprise that meanwhile also preformed zirconia crowns are available on the market.
However, due to material properties, the side walls of these crowns are not flexible and therefore no undercut design is possible. Further, compared to the stainless steel crowns, a different and more invasive tooth preparation without or with a very limited possibility of undercut retention is needed, which leads again to a more careful cementation technique. Solutions of roughening the inner surface of the crown, e.g. sandblasting or surface retention structure should help to support the cementation of the crown, but in sum could not replace the undercut feature.
It has also been suggested to manufacture preformed crowns out of polymer materials.
In this respect, U.S. Pat. No. 8,651,867 B2 (Zilberman) describes a dental crown configured to be readily mountable in a patient's mouth as part of a treatment of primary teeth and permanent molars, the dental crown having a natural appearance and color of a vital tooth and consisting of a thermoplastic material layer configured to define a tooth shaped top surface and flexible side surfaces.
As suitable thermoplastic materials polymers selected from polyacetal, polyacrylate, polymethacrylate (PMMA), polyaryletherketone (PAEK), polyetherketon (PEK), polyetheretherketon (PEEK), polyetherimide (PEI), polyethersulfone (PES) and polysulfone (PSU) are suggested.
US 2004/0161726 A1 (Saito et al.) describes a crown prosthesis having wear resistance and an aesthetic property comprising a polymer of a mixture of a polymerizable compound having an unsaturated double bond, a filler and a polymerization initiator, and having an outer shape resembling a tooth and a space to be filled with a dental composite resin between an inner surface thereof and an abutment tooth.
However, the workflow needed for applying polymer material based preformed crowns does not really differ from the workflow needed for applying preformed zirconia crowns, even if a more flexible material is used.
The commercially available polymer based preformed crowns show wall thicknesses of more than 500 μm, which gives them not enough flexibility and therefore no undercut design options as an additional support for cementation.
The fact that the material properties do not allow thinner walls makes the walls a limiting factor for the design options.
To nevertheless ensure a sufficient adhesive fixation of these crowns to a prepared tooth surface, an adhesive cementation, ideally with pretreatment of the inner surface is recommended or required, even if surface roughening is applied. This makes the whole prosthetic procedure more complicated and expensive. Generally, for crowns and bridges different cementation techniques are available.
These can be divided into clusters like temporary cementation (e.g. RelyX™ TempNE/E from 3M Oral Care; 3M ESPE), conventional cementation (e.g. Ketac™ CEM or Ketac™ CEM Plus from 3M Oral Care; 3M ESPE), self-adhesive resin cementation (e.g. RelyX™ Unicem from 3M Oral Care; 3M ESPE) or adhesive resin cementation (e.g. RelyX™ Ultimate from 3M Oral Care; 3M ESPE).
In general, the cementation needs to be durable over the life time of the indication, which could be achieved either due to chemical bonding or mechanical retention or a combination thereof.
The choice of the used cement or the general cementation technique for a specific indication is therefore influenced by the material of the restoration, the indication itself, the preparation technique, but also cost and esthetic plays a role.
For a fast and easy chairside workflow with preformed crowns, e.g. pediatric dentistry a fast and easy cementation technique is not only desired but required.
For this reason for the fixation of stainless steel crowns conventional cementation techniques are used. Those cements do not only have an easier workflow, but also are cheaper than self-adhesive resin or adhesive resin cements. Moreover they are more moisture tolerant and robust against blood and saliva than self-adhesive or adhesive cements.
This technique is the dominating one in pediatric dentistry, due to the time saving chairside workflow and the fact, that an individually designed crown is not necessarily needed.
In the end, the broad use of this type of crowns is the result of its easy workflow at the dentist chair. This easy workflow is very important to shorten the chairside time.
One important factor to enable this easy workflow is the provision of a material which enables an undercut design of the crown in the marginal area, doable due to thin and therefore flexible side walls. Especially for primary molar teeth, an undercut design leads to an easy, fast and less invasive preparation technique of the tooth stump, which enables a so called snap-on effect of the crown to the stump.
The snap-on effect ensures also an easy placement of the crown. Moreover the undercut design and preparation technique results in a macroscopic mechanical retention of the crown, which supports the cementation of the crown significantly.
WO 2007/098485 A2 (Nusmile) describes a preformed dental crown with a center surface, a circumferential surface transitioning from and integral with the central surface wherein the circumferential surface includes a taper toward a gingival end and wherein said taper has a thickness ranging from 0 to 0.5 mm at a gingival edge to at least 1.0 mm proximate the transition to the center surface.
WO 2008/033758 A2 (3M) describes a solid dental crown including a self-supporting solid hardenable preformed dental crown having an external crown shape defined by an external crown shape defined by an external crown surface.
US 2007/0196792 A1 (Johnson et al.) describes a prefabricated dental crown being tooth colored and having an undercut. Materials which are said to be useful for manufacturing the prefabricated dental crown are thermoplastic resins such as polyacetal, polyacrylate, polyamide, polyaryletherketone, polyetheretherketone (PEEK), polyetherimide, etc.
However, none of the solutions suggested in the prior art is completely satisfying. All these types of crowns do have drawbacks from different aspects, mainly related to a more complex and time consuming workflow but also due to durability and/or esthetic.